The present invention relates generally to wireless communication networks, and more particularly, to a system and method for generating a plurality of pilot beacons to facilitate a precise hard handoff.
The success of code division multiple access (CDMA) wireless communication in the recent years has brought quality service to many mobile users. With the advancement of the technology and the demand of the market, it is foreseeable that more private CDMA networks will be installed in addition to existing public CDMA networks such as the Sprint PCS network by Sprint, Inc. Major corporations or institutes may have the desire to establish private CDMA networks on their campuses to provide high quality and more economical wireless service to their employees or guests. However, such campuses are often heavily populated with mobile units such as cellular phones. To worsen the situation, the size of the campus is relatively small so that it may reside entirely in one cell of a larger network.
One major feature of CDMA technology that differentiates it from other wireless communication technologies is its use and reuse of specific frequencies. Although it is a technical edge over other communication technologies such as time division multiple access, it makes the campus scenario described above an undesirable xe2x80x9chot spotxe2x80x9d since a comparatively large number of users may have to be serviced by one sector of the cell using the same frequency. This possible xe2x80x9ctraffic jam on the communication highwayxe2x80x9d reduces the quality of the wireless services provided to the mobile units on the campus.
As a result, it may be desirable for a private CDMA network on a campus to use a carrier frequency different from the surrounding networks. And to further improve the quality of the services on the campus, numerous micro cells may be used, each of which having a comparatively small radius (typically less than one hundred meters). In contrast, a xe2x80x9cmacro cellxe2x80x9d for a standard CDMA network surrounding the campus may have a radius around 3 to 5 kilometers. Consequently the signal interferences between the surrounding network and the private network on the campus, as well as the signal interferences among the micro cells in the private network, are greatly reduced because different carrier frequencies are employed. However, for providing continuous wireless service to a mobile unit (MU) when it enters or leaves the campus, a hard handoff must occur between the private network and the outside network.
There are several difficulties associated with the above described hard handoff. For one, an active wireless service such as an ongoing telephone call should not be interrupted or dropped due to the change of the carrier frequency when carrying out such a hard handoff. In addition, the service provided should be properly billed, potentially to different service providers, according to the use of the service of the different networks. Thus, for example, when a mobile user exits the campus, he should be able to continue his conversation on the MU, even though the MU needs to switch from the private network to the surrounding network. Such a transition must be performed in a relatively small border area between the campus and outside, and vice versa when the mobile user enters the campus.
Unfortunately, various problems exist for precisely performing the handoff at the border area. First, soft handoffs do not work well in this situation. It is known in the art that the soft handoff for CDMA technology has been studied and developed greatly. But in order to take advantage of a soft handoff, only one carrier frequency can be involved. Soft handoffs are thus most useful for switching the wireless service between two Base Transmission Stations (BTS) using the same carrier frequency. For the situation discussed above, since at least two carrier frequencies are involved, the soft handoff scheme would be hard to deploy. For example, in order to use the soft handoff scheme, as it is known in the art, the only alternative is to configure a tier of cells for providing a soft handoff zone between the campus and the surrounding network. This would dramatically increase the cost of installing such a private network since additional, expensive infrastructure equipment must be installed. Moreover, significant redesign of the surrounding network is also required to accommodate this change, which adds further costs to the private network. Therefore, a soft handoff is economically infeasible and a hard handoff design is needed.
There is still a problem associated with using a conventional hard handoff scheme, such as a round trip delay handoff, in a small area. As it is known in the art, the round trip delay handoff can detect the motion of a MU only when it moves for more than 250 meters. This does not work for the campus scenario mentioned above because the campuses themselves may have a radius of less than 500 meters. The handoffs thus can not be guaranteed to happen at or near the entrance or the exit of the campus since the border area between the campus and the surrounding network is probably only about 25 meters in length.
Therefore, what is needed is an economical and reliable system and method to direct a precision hard handoff to accommodate the carrier frequency change between a relatively small private CDMA network and its surrounding networks.
In response to the above described problems and difficulties, a technical advance is provided by a unique method and system for providing a precision hard handoff between two networks. In one embodiment, the two networks include a first wireless communication system for serving a defined area, such as a campus, at a first frequency and a second wireless communication system adjacent to the first system and operating at a second frequency. A first pilot beacon distributor is positioned at a transition point of the campus for directionally transmitting a first pilot beacon signal into the campus. When a mobile unit operating on the second frequency crosses the transition point into the campus, it receives the first pilot beacon signal and it shifts operation to the first frequency.
In some embodiments, a second pilot beacon distributor is also positioned at the transition point for directionally transmitting a second pilot beacon signal away from the campus. As a result, when a mobile unit operating on the first frequency crosses the transition point out of the campus, it receives the second pilot beacon signal and it shifts operation to the second frequency.
In another embodiment, a plurality of pilot beacon distributors are installed at each entrance or exit of a campus and are connected by fiber optical cables to a radio frequency (RF) beacon source which may remain at a central location on the campus. Only one beacon source is needed for each carrier frequency covering the entrance or exit area. The pilot beacon distributors have directional beacon antennas that transmit the pilot beacons to help trigger a hard handoff for any MU moving in or out of the campus.
In some embodiments, a first pilot beacon distributor is used to direct hard handoff from a carrier frequency for the private network on the campus, e.g., F1, to a target carrier frequency, e.g., F2, of the outside network for outgoing traffic, and a second pilot beacon distributor is also installed close to the first pilot beacon distributor for steering incoming traffic to hard handoff from F2 to F1.
Some embodiments may provide a directional non-beacon antenna that does not transmit the pilot beacon. The directional non-beacon antenna may be coupled with the above mentioned directional beacon antenna in the pilot beacon distributors. The directional beacon antenna may have a large front-to-back ratio, while the non-beacon antenna may not have a significant front-to-back ratio.
These embodiments prevent a ping-pong phenomenon between the first and the second pilot beacon distributors. For example, a ping-pong phenomenon may occur when both pilot beacon distributors detect the motion of a MU and both decide to trigger a handoff to its target carrier frequency. In this example, the MU will bounce between the two frequencies (F1 and F2) without actually being able to settle on a right one.
These embodiments also prevent call drops from occurring if the signal strength of a target carrier frequency is too weak at the locations where the handoff is desired. For example, a handoff may become unreliable since the target network can not effectively xe2x80x9ctake overxe2x80x9d the service.
These embodiments are also specifically designed to maintain a proper signal-to-noise ratio (SNR). If the SNR under the current carrier frequency changes too fast, it could cause a call drop as well and the handoff can never be completed.
In order to accommodate complex CDMA networks where multiple frequencies are used both for the surrounding network and the private network on the campus, a multiple-beacon generator may also be used. Instead of installing a pilot beacon unit for each frequency, the multiple-beacon generator can produce copies of a pilot beacon at different carrier frequencies.